This invention relates to signal processing and more particularly to a method and apparatus for transmitting control information over an audio data stream. This invention finds applications in digital communication systems, such as a digital cellular system or a Voice-over-IP (VoIP) network, in particular codec bypass capable systems that can selectively de-activate encoding/decoding functions in the connection to provide improved speech quality.
In a digital communication system such as a wireless network or a VoIP network, an audio signal may be processed by a series of speech encoders and decoders as it is transmitted from one endpoint to another. In the example of a cellular mobile-to-mobile connection, the audio signal is first encoded by a speech encoder at the first mobile telephone and transmitted in an encoded format to a base transceiver station of a cell site where it is transferred to the base station controller servicing that cell site. At the base station controller, the encoded speech information is processed by a compatible speech decoder that converts the compressed speech stream into PCM samples. The PCM samples are then transported over the landline network, such as the PSTN, toward the base station controller servicing the cell site communicating with the other mobile telephone. At the second base station controller, the PCM speech samples are again processed by a speech encoder. The encoded information is sent from the base transceiver station of the cell site to the second mobile telephone where the compressed speech stream is converted one more time by a speech decoder into PCM samples that can be used to generate an audio signal.
In this example, codecs on both sides of the mobile-to-mobile call are connected in tandem, which is known to degrade the speech quality as a result of the successive encoding/decoding of the audio signal.
The xe2x80x9ccodec bypassxe2x80x9d technique alleviates this problem. During a connection, when the codecs at the base station controllers are made aware of their mutual existence, they are switched off such that the encoded speech information arriving at the first base station controller flows into encoded format through the PSTN and arrives as such at the second base station controller. This procedure eliminates one decoding operation of the speech signal at the first base station controller and one re-encoding operation of the signal at the second base station controller. As a result, the audio quality is significantly improved.
For additional information on the xe2x80x9ccodec bypassxe2x80x9d technique, the reader is invited to refer to the U.S. Pat. No. 5,768,308 granted to the same assignee that describes the process in great detail. The contents of this document are hereby incorporated by reference.
The codec in one base station controller can switch to the bypass mode as a result of an in-band hand-shaking operation with the companion codec in the other base station controller. Transmitting control information from one codec to the other over the audio data stream allows this hand-shaking operation to take place. The control information is transmitted by bit stealing. This is effected by inserting in selected PCM samples bits from the control information signal. The control information bit is inserted at a position of pre-determined significance in the sample. Since this insertion mechanism destroys the audio information bit at the position of predetermined significance, it is desirable to carry out the insertion at the lowest possible bit significance position. In a specific example, a T1 connection between the two codecs may require signalling information to be embedded in some speech samples at the least significant bit position, where this signalling information is necessary for the transmission facility linking the two codecs. Accordingly, the control information is sent over the next available bit position, specifically bit significance position 1. This effectively destroys the audio information at bit significance position 1 in every sample carrying control information.
Although the above described technique allows obtaining an overall good speech quality, it would be desirable to effect the transmission of the control signal while avoiding as much as possible the loss of the audio information in the bit position in which the control information bit is carried.
The present invention provides a device and a method for transmitting control information over an audio data stream. This device can be used in a digital communication system implementing the xe2x80x9ccodec bypassxe2x80x9d feature, for example a Time Division Multiple Access (TDMA) system or a Code Division Multiple Access (CDMA) system. The device comprises a first input for receiving the audio data stream, the audio data stream being characterised by a plurality of data blocks, each block including a plurality of bits arranged in different significance positions. The device also includes a second input for receiving a control information signal including a plurality of bits. A control unit is coupled to the first and to the second inputs to insert in selected data blocks of the audio data stream bits from the control information signal. More particularly, the control unit is operative to shift a bit at a selected significance position in a data block to a lower significance position and insert a bit from the control signal at the vacated selected significance position of the data block. More than one bit can be shifted to lower significance positions if more positions in the data block are to be vacated so that more than one control information bit can be inserted in the data block.
The resulting signal is then released from the device for transmission to a remote location. In a specific example, the data block is a PCM sample.
Preferably, at the receiving end of the transmission, the multiplexed audio data stream and the control information signal are separated. The operation involves, for selected ones of the data blocks, extracting a bit at a first pre-determined significance position and shifting the bit(s) at significance position(s) below the first pre-determined significance position towards the first pre-determined significance position. The extracted bits from the selected data blocks can then be used to assemble the control information signal, while the selected data blocks with the shifted bits can be used in the formation of the audio stream suitable for further processing.
The bit shifting allows preserving an audio information bit in the data block, that would be lost under prior art bit stealing transmission techniques. Objectively, a bit loss will also occur in some samples that receive, in addition to a control information bit a signalling information bit. This event does not occur very often as most of the data blocks selected for carrying information other than audio information carry either control information or signalling information but not both. Note that, in a specific example, the transmission facility linking the two codecs may be a single T1 link formed of a series of T1 connections having unsynchronised symmetry. In such a situation, the multiple T1 signalling has a higher chance of coinciding with the control bits, such that a particular data block may be selected to carry both control and signalling information.
The invention also provides a device and a method for demultiplexing a compound digital signal into audio data and a control information signal.
Yet, the invention also provides a data structure containing audio and control information implemented over a machine readable data transmission medium, said data structure having a bit pattern XYZ, where X is a bit set including at least one audio information bit, Y is a bit set including at least one control information bit and Z is a bit set including at least one audio information bit, when bit set Z is shifted up by the number of bits forming the bit set Y a bit word XZ being formed that is characterised by bits arranged in predetermined significance positions, the bits in the word XZ communicating in conjunction with one another audio information. In a specific example, the bit sets Y and Z contain each a single bit.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.